There are a few number of military and commercial situations that would benefit from the availability of a portable magnetic sensing system to perform real-time, point-by-point “Detection, Localization and Classification” (DLC) of magnetic objects (or “targets”). For example, this type of system is needed when searching for hidden and buried “unexploded ordnance” (UXO), e.g., bombs, artillery shells, mines, etc., that are typically found in former defense sites and past/present war zones.
As used herein, the term “real-time” means that the time interval between sensing system's measurement and display of target DLC parameters (i.e., the target's presence, location and magnetic signature) should be on the order of a few seconds or less. The term “point-by-point” indicates that the portable sensing system should be able to provide a real-time indication of DLC parameters at each and every point in which the sensing system can detect the target. Further, the portable magnetic sensing system should be easily carried and operated by a single individual and be generally adaptable to a wide variety of highly mobile autonomous sensing platforms. However, because of the limitations of prior art magnetic sensing technologies, the prior art does not provide an easily portable magnetic sensing system that can provide effective real-time, point-by-point DLC of magnetic targets.
The limitations of prior art technologies primarily relate to their inability to adequately address the following technical factors that affect magnetic anomaly sensing:
(i) The magnetic field (B) that emanates from a magnetic object/target is a three-dimensional vector quantity that, in accordance with the well known “dipole approximation” is a function of six quantities; namely, the components of: a) the three-dimensional vector position (r) of the field point with respect to the target; and b) the three-dimensional vector magnetic moment or magnetic signature (M) of the object's ferrous materials. The magnitude of B rapidly decreases as the inverse cube of distance from the target, i.e., B∝r−3. At sensor-target distances of just a few meters, the magnetic field B of a typical UXO-sized object typically is very small (i.e., less than or equal to 10−9 tesla or 1 nano tesla (nT) or less). Thus, B rapidly changes with distance and constitutes a relatively short-range magnetic anomaly that typically is many orders of magnitude smaller (e.g., less than 1/50,000) than the relatively large and constant magnetic field of Earth (BE).
(ii) The magnetic field of Earth BE is a locally fairly constant vector quantity whose magnitude typically is (depending on latitude) about 50,000 nT. Thus, if a magnetic sensing platform changes its orientation by 180°, it's vector field sensing elements could measure a total change of about 100,000 nT. The platform-orientation-dependent changes in measured field components constitute platform “motion noise” that are many orders of magnitude larger than the target's magnetic anomaly field (or signal).
(iii) In order to use a magnetic sensing system to perform point-by-point determination of the position r (i.e., the vector quantity indicative of range, elevation and bearing to a target) and the magnetic signature M of a magnetic object, the system must measure sufficient magnetic anomaly field data at each point in space nearly simultaneously to solve for the six unknown components of r and M. Therefore, in order to perform point-by-point DLC, a mobile sensing system must accurately and nearly simultaneously measure and discriminate at least six independent magnetic anomaly field variables that are convolved at every point with the tens of thousands of times greater magnetic “noise” from the Earth's field.
Thus, in order to perform effective localization and classification, a mobile magnetic sensing system has two major interrelated technical requirements. First, the system must have a wide dynamic range on the order of 105 (or more) to discriminate multiple sub-nT signal components that are convolved within the Earth's 50,000 nT field. Second, the system must reduce the effects of sensor “motion noise” that can arise when the sensor system changes orientation in the Earth's field. Generally speaking, two magnetic sensing technologies have been developed to address the technical difficulties involved in detection and discrimination of magnetic anomaly fields. They are known as scalar total field magnetometry and tensor gradiometry. However, it is well known in the art that scalar total field magnetometers essentially measure, only one component of an object's magnetic anomaly field, i.e., the component that is parallel to the Earth's magnetic field direction. The resulting limited data sets provided by scalar magnetometry preclude the use thereof for real-time, point-by-point DLC of magnetic objects.
Tensor gradiometers measure the rate of change with distance (i.e., the gradient) of magnetic fields. Since the gradient of BE is fairly small (i.e., typically about 0.02 nT per meter), a gradiometer is able to discriminate a small magnetic anomaly field gradient within the large Earth's background field. Thus, tensor gradiometers can, in principle, perform point-by-point DLC. However, in practice, limitations of their sensor system embodiments, data sets and DLC methods cause the performance of prior art tensor gradiometers-based sensing systems to be degraded by changes in orientation of the sensing system's platform. Changes in sensor system orientation change the gradient component measurements and generate motion noise that can greatly reduce the range, accuracy and effectiveness of a conventional prior art tensor gradiometer. Therefore, in practice, conventional tensor gradiometers require sensor platform motion to be constrained to straight line trajectories with very little change in orientation. Such constraints on motion greatly reduces the range of potential applications for conventional gradiometers.
U.S. Pat. No. 6,476,610 (i.e., the “610 patent” as it will be referred to hereinafter) discloses a novel magnetic tensor gradiometer system and signal processing methods based on a magnetic “Scalar Triangulation and Ranging” (STAR) concept. The STAR concept was created to address the limitations on the practical mobility of conventional gradiometry and provide a more effective, motion noise restraint magnetic sensing technology for maneuverable sensing platform.
U.S. Pat. No. 6,841,994 (i.e., “the '994 patent” as it will be referred to hereinafter) discloses a multi-toner STAR technology that uses three-dimensional arrays of triaxial vector magnetometers to develop unique, rotationally invariant (i.e., platform motion noise resistant) scalar “contractions” (or magnitudes) of magnetic gradient tensors. The tensor contractions provide a motion noise resistant data base that the STAR method uses to perform point-by-point triangulation of a magnetic target's location and to determine the target's magnetic signature. However, the '610 and '994 patents do not specifically disclose means and methods for application of the STAR technology to address the technical challenges involved in the development of a man-portable magnetic sensing system for real-time, point-by-point DLC of magnetic objects.